Nonstoichiometry and Defect Chemistry in Praseodymium-Cerium Oxide

Praseodymium cerium oxide (Pr_xCe_{1 – x}O_{2 –} ) is a mixed ionic-electronic conductor with high levels of nonstoichiometry under oxidizing conditions resulting from reduction of Pr^{4 +} to Pr^{3 +}. Coulometric titration measurements performed on (Pr_xCe_{1 – x}O_{2 –} ) with x = 0.2 are generally consistent with those derived from electrical conductivity measurements. Nevertheless, a somewhat larger degree of nonstoichiometry measured via coulometric titration implies that non-charged defect species may be significant in the system.     

Ceria-Zirconia Composite Electrolyte for Solid Oxide Fuel Cells

Ceria-zirconia-ceria sandwich structured composite filmelectrolytes were designed in order to offer high ionic and low electronicconductivity electrolyte films. Calculation of oxygen potential profile inthe composite film electrolyte indicates that a very thin zirconia filmkills the electronic current of ceria without affecting the ionicconductivity. The composite films were successfully prepared by a co-fireprocess. The main problem was the fact that the ceria and zirconia greenfilms had different shrinkage behaviors. Successful co-firing was achievedby controlling the temperature program and amount of binder. De-laminationbetween yttria stabilized zirconia(YSZ) and gadolinia doped ceria(GDC)layers was overcome by the formation of a solid solution phase at theinterface of the two films. The resultant composite films, however, showedpoor electrical conductivity compared with theoretical values. The formationof a solid solution phase can have a negative effect on compositeelectrolytes. Also, the composite film is unsatisfactory in terms ofmechanical strength. This could be due to the lattice expansion in reducingatmospheres, thermal expansion coefficient mismatch, or the intrinsicweakness of the ceria texture.     

Sintering Behavior of Cobalt Oxide Doped Ceria Powders of Different Particle Sizes

The effect of cobalt oxide as sintering aid for CeO₂ and Ce_0.9Gd_0.1O_1.95 powders was studied as a function of initial powder particle size. The sintering effect of cobalt oxide, measured as the difference in temperature of maximum shrinkage rate between undoped and 1 cat% cobalt oxide doped powders, decreased with initial particle size. Almost no effect was found for the smallest particle sizes investigated (d < 10 nm) whereas the maximum shrinkage rate temperature decreases by more than 200C for particles d > 100 nm. For the smaller particles, only doping of much higher concentration produces significant decreases in the temperature of maximum shrinkage rate. This observation suggests that the effectiveness of sintering depends on the specific surface area of the starting powder and points at the decisive role of the doping method used. Dense microstructures with average grain sizes smaller than 100 nm are obtained by doping very fine powders with cobalt oxide.     

SOFC电解质薄膜制备技术研究进展

针对固体氧化物燃料电池(SOFC)电解质薄膜制备技术与电池性能研究进展进行了综述。SOFC是高效率的电源装置,但其效率严重依赖于电解质层的薄膜化。对比分析了目前电解质薄膜制备中使用的干压法、流延法、丝网印刷、浸涂法、溶胶凝胶法、电泳沉积、物理气相沉积、化学气相沉积、等离子喷涂等技术的优劣之处,并探讨了不同制备技术对电池综合性能的影响。     

SOFC电解质与电极材料的燃烧合成及性能

采用柠檬酸-硝酸盐燃烧合成了CeO2基SOFC的电解质与电极材料,主要使用X射线衍射(XRD)和扫描电镜(SEM)初步研究了烧结温度对掺杂CGO(Ce0.8Gd0.2O1.9)、YSZ(Zr0.92Y0.08O2)、NiO、LSCF(La0.6Sr0.4Co0.2Fe0.8O3-δ)相结构的影响以及CGO、LSCF的失效机理.实验结果表明,经过时效处理后的CGO的晶界处以及晶粒表面都生成了大量的小孔,LSCF的失效可能源于其表面结构和相组成的变化.     

Defect chemistry and transport properties of Pb(Zr1/2Ti1/2)O3

Abstract

In order to gain insight into the degradation mechanisms associated with ferroelectric thin films, such as fatigue and imprint, an understanding of the defect chemistry and transport properties of the material is needed. In this study several complimentary techniques have been used to either measure or calculate indirectly the various thermodynamic parameters governing defect formation and transport in Pb(Zr_1/2Ti_1/2)O₃, (PZT). By combining the results of DC equilibrium conductivity, thermoelectric power and the sealed cell techniques, “constant composition oxygen activity” and “constant composition conductivity,” values for the oxidation enthalpy (ΔH_ox), hole trapping energy (E _A) and the enthalpy of motion for holes (ΔE _A) have been determined to be ?0.49 eV, ≤0.9 eV and ≥0.1 eV, respectively. From these results, it is apparent that PZT is an oxygen excess p-type semiconductor in the experimental regime of 500°–700°C and P(O₂) ≥ 10^?4 atm. Furthermore, the results indicate that there is a significant concentration of trapped holes at high temperatures and hole conduction appears to be an activated process (i.e. small polaron conduction).     

固体氧化物燃料电池用YSZ电解质纳米粉体的湿法工艺进展

综述了固体氧化物燃料电池电解质材料———钇稳定氧化锆(YSZ)纳米粉体的各种湿化学制备方法,包括溶剂蒸发法、共沉淀法、溶胶-凝胶法、水热法和低温燃烧合成工艺等,比较了它们各自的特点,并对这些方法进行了评述和展望。     

Nonstoichiometry and Electrical Conductivity of Nanocrystalline CeO{2 - x}

We synthesized dense CeO polycrystals of 10 nm grain size and characterized their electrical conductivity, in order to determine whether the defect properties of nanocrystalline solids fundamentally differ from those of conventional materials. The nanocrystals exhibit enhanced electronic conductivity, greatly reduced grain boundary impedance, and a heat of reduction more than 2.4 eV lower per oxygen vacancy compared to their coarse-grained counterparts. We propose that defect formation at low energy grain boundary sites is responsible for these properties, and that nanocrystalline oxides represent bulk materials possessing the defect thermodynamics of interfaces.     

SOFC电解质薄膜YSZ制备技术

综述了固体氧化物燃料电池 (SOFC)电解质薄膜YSZ的制备方法 ,通过EPD、APS、VPS、EVD、Sol Gel和流延、轧膜、丝网印刷等方法都可以制备YSZ薄膜。从成膜质量上比较 ,APS、VPS、EVD等方法制得的YSZ薄膜气密性好 ;从生产成本上分析 ,由高到低依次是VPS、APS、EVD、Sol Gel和流延、轧膜、丝网印刷等陶瓷成膜方法。目前 ,制备YSZ电解质薄膜使用较多的是APS、EVD ,最适合大工业化生产、成本低廉的是陶瓷薄膜成型方法 ,市场前景广阔     

用于燃料电池的氧化锆薄膜制备方法进展

固体氧化物燃料电池(SOFC)具有高效率、低污染等优点,氧化钇稳定氧化锆(YSZ)是目前用于SOFC的最成功的电解质材料,为了减小高温运行带来的困难,应用中需将YSZ制成薄膜。综述了制备YSZ 薄膜的各种方法,其中包括化学气相沉积(CVD)、电化学气相沉积(EVD)、溶胶-凝胶法(Sol-gel)和喷雾热解法等化学方法;物理气相沉积技术(PVD)和喷涂技术等物理方法;以及电泳沉积法(EPD)、注浆成型法和离心浇铸法等陶瓷成型方法,介绍了近年来采用上述方法制备的YSZ膜的性能及其用于SOFC电池研究取得的实验结果,最后评述了这些方法各自的特点。     

固体氧化物燃料电池研究进展

固体氧化物燃料电池是直接将化学能转化成电能的全固态化学发电装置,具有能量转化效率高,无腐蚀,燃料适应性强和寿命长等优点。介绍了固体氧化物燃料电池的阳极、阴极、电解质等核心部件,介绍了固体氧化物燃料电池存在的问题,并对未来发展前景进行了展望。     

固体氧化物燃料电池的数学模型及自适应神经模糊辨识模型的研究

固体氧化物燃料电池(solid oxide fuel cell,SOFC)是21世纪最有生命力的发电技术之一。文章从SOFC实际应用的角度出发,应用改进的自适应神经模糊推理系统(adaptive neural fuzzy inference system,ANFIS)对SOFC建立了 负载稳定和负载变化2种情况下的电特性模型。由于数据来源不足,首先根据SOFC的工作原理,运用电化学、流体动力学等学科理论,建立SOFC的数学模型,基于该数学模型获取ANFIS辨识模型的训练和预测数据。仿真结果显示了改进的ANFIS技术对SOFC系统的建模和控制具有一定的实用价值。     

Interface Defect Chemistry and Effective Conductivity in Polycrystalline Cerium Oxide

Polycrystalline cerium oxide exhibits increasing electronic and decreasing ionic conductivity upon reduction of the grain size. In the present study, the origin of this effect was examined. Temperature-programmed reduction (TPR) and oxygen titration measurements on nanocrystalline cerium oxide revealed a large excess oxygen deficiency associated with the surface. Using a two-phase model for the combined system of the bulk phase in equilibrium with a surface layer, this enhanced oxygen deficiency could be explained by a reduced binding energy of surface oxygen ions in agreement with results from atomistic computer simulations. The model also revealed that this segregation of oxygen vacancies is the origin of an intrinsic space charge potential. Translating this effect to polycrystalline cerium oxide and taking into account the segregation of dopants and the accumulation/depletion of charge carriers, it was possible to model the grain size dependence of electrical conductivity and thermopower of polycrystalline cerium oxide. A straightforward 1-dimensional numerical model and a change from Boltzmann to Fermi-Dirac statistics allowed to calculate the conductivity of heavily doped polycrystalline cerium oxide for grain sizes in the range of 5–10,000 nm and acceptor concentrations up to 20%. Using this approximation, the effect of grain size on mixed ionic/electronic conductivity and the electrolytic domain boundary was investigated.     

Defect Chemistry of SrBi2Ta2O9 and Ferroelectric Fatigue Endurance

The defect chemistry and charge transport properties of doped and undoped SrBi₂Ta₂O₉ (SBT) were studied by making 4-point dc equilibrium electrical conductivity, thermopower, ionic transport number, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements. Results of high temperature equilibrium dc conductivity, thermoelectric power, and transport number measurements as a function of oxygen activity in the temperature range 650–725°C revealed that undoped SBT displays a broad centrally located plateau of ionic conductivity with an activation energy of mobility of 0.94 eV and a prominent upturn at high oxygen activity, caused by p-type conductivity. The effects of acceptor and donor dopants are consistent with a 1–2% net acceptor excess in the undoped compound. It has been observed that there is substantial (several percent) cation place exchange between the Sr²⁺ and Bi³⁺ in SBT. It is proposed that the net acceptor excess in undoped SBT consists of disordered Sr²⁺ substituting for Bi³⁺ in the fluorite-like bismuth oxide layers which are locally compensated by oxygen vacancies. The formation of the net donor excess by disordered Bi³⁺ substituting for Sr²⁺ in the perovskite-like layers does not manifest itself as n-type conductivity behavior, because the band gap is large and the mobility is highly thermally activated. The superior intrinsic ferroelectric fatigue endurance of SBT is attributed to the lack of mobile charged defects in the perovkite-like layers which create the ferroelectric response of the compound. The metallic bismuth presence on the surface of undoped SBT, as revealed by qualitative XPS measurements, is believed to result from long exposure to the highly reducing conditions in the XPS system.     

Ce0.8Y0.2O1.9电解质的制备及性能

以金属硝酸盐为原料,碳酸钠为共沉淀剂制备了电解质材料Ce0.8Y0.2O1.9(20 YDC),并对其性能进行了研究。XRD测试表明碳酸盐前躯体经600℃煅烧处理即可得到萤石结构氧化物,TEM观察煅烧后的粉体为分散性较好的球形纳米颗粒。对700℃煅烧的粉体冷压成型后进行烧结性能的研究,SEM分析和致密度的测量表明经1 400℃烧结4 h致密度能达到95%以上。利用两端子交流阻抗谱法在400～700℃空气气氛中测量了电解质的电性能,1 400℃烧结的20YDC电解质在700℃和600℃的电导率分别为0.04、0.015 S/cm,电导活化能为0.85 eV,表明其有望应用于中低温固体氧化物燃料电池。     

固体氧化物燃料电池失效快速判断机理探讨

在简单综述了导致固体氧化物燃料电池(Solid Oxide Fuel Cell,SOFC)失效的主要原因的基础上,探讨了采用实验室模拟加速的实验研究和材料损伤演化动力学与计算机模拟的理论研究来对SOFC的失效进行快速判断的可行性。     

Modeling and Characterization of Electrical Transport in Oxygen Conducting Solid Electrolytes

A model for the electrical conductivity in acceptor-doped oxides which involves an association between the acceptor-dopants and oxygen vacancies resulting in donor centers is considered. The model relates the behavior of the electrical conductivity with the temperature, ambient atmosphere and band structure. The predictions of the model are compared to experimental data for ZrO₂:16% Y and SrCeO₃:5% Yb oxygen conductors and some band structure parameters have been determined.     

锥管状LSGM电解质支撑的SOFC的研制

以甘氨酸-硝酸盐燃烧法合成的La0.9Sr0.1Ga0.8Mg0.2O2.85(LSGM)为原料,阿拉伯树胶为分散剂,采用注浆成型法制备长度约为10 mm、壁厚为0.2 mm、大口端直径约为8.5 mm的致密锥管状LSGM电解质管。以质量比为7∶3的Ni O-Gd0.1Ce0.9O1.95(GDC)、Sm0.5Sr0.5Co O3(SSC)-GDC分别为阳极、阴极材料,组装单体固体氧化物燃料电池(SOFC)。以加湿氢气(含3%H2O)为燃料、空气为氧化剂,电池在600℃时的最大输出功率密度约为87 m W/cm2。交流阻抗谱分析表明:影响电池性能的主要因素是电解质的欧姆电阻。     

CeO2基中低温固体电解质材料研究进展

用稀土或碱土氧化物掺杂的CeO2在较低的温度下具有较高的氧离子电导率,是用作中低温固体氧化物燃料电池(SOFC)最合适的电解质材料.综述了近年来对以掺杂的CeO2作电解质的SOFC的性能的研究情况,这些电解质材料主要是Ce0.8Sm0.2O1.9,Ce0.8Gd0.2O1.9,Ce0.8Y0.2O1.9等,目前已达到的最好性能是在500 ℃下,以Ce0.8Sm0.2O1.9作电解质的单电池输出功率为403 mW/cm2.CeO2基电解质材料不易烧结,其烧结温度一般在1 300 ℃以上.还对CeO2基电解质材料的一些制作工艺进行了介绍,包括固相反应法,溶胶-凝胶法,热液合成法.     

磷灰石结构电解质La_(10-x)Si_6O_(27-1.5x)的合成及电性质

以La_2O_3和SiO_2为原料,采用凝胶注模工艺合成了具有磷灰石结构晶相的固体电解质材料La_(10-x)Si_6O_(27-1.5x)(x=0,0.5).通过X射线衍射光谱法(XRD)对所得化合物的结构进行表征.在300～800 ℃的温度范围内,利用电化学阻抗谱详细研究了La_(10-x)Si_6O_(27-1.5x)陶瓷的烧结温度和La元素的含量对La_(10-x)Si_6O_(27-1.5x)电导率的影响.结果表明:陶瓷样品的烧结温度对其电导率的影响较大,La_(10)Si_6O_(27)的电导率随着陶瓷样品烧结温度的上升而提高,1 650 ℃烧结的La_(10)Si_6O_(27)在800℃电导率为3.46×10~(-2) S/cm,约是1 550 ℃烧结样品的6倍.La_(9.5)Si_6O(26.25)的电导率随着陶瓷样品烧结温度上升呈先上升后下降的变化趋势,当烧结温度高于1 600℃时,晶粒和晶界内有大量未知相析出,这可能是导致其电导率反而下降的直接原因.